1. Field of the Invention
The present invention relates a method and apparatus for calculating strain and fatigue damage. More specifically, the invention relates to a method and apparatus for calculating strain and fatigue damage in materials based on observing the deformation of a geometric shape associated with (for example by affixing to, identifying on, or embedding in) the body under study, coupled with the use of finite element analysis principles and principles of mechanics of materials.
2. Related Art
There have been many technical efforts made towards measuring and/or calculating strain and determining fatigue damage in materials. “Measuring” as used herein means determining a magnitude with reference to a standard of measurement; and “calculating” as used herein means determining a value using a mathematical model of a physical model of a phenomenon or a phenomenological equation. Examples of such technical efforts are disclosed in U.S. Pat. No. 4,591,996 to Vachon and Ranson, Japanese application No. JP 57125803 for “Method for Measuring Strain of Sample Surfaces,” Japanese patent publication No. JP 2002267422 for “Method and Device for Crack Detection,” U.S. Pat. No. 4,008,960 for “Photoelastic Strain Gauge Coating and Method of Using Same,” European Patent Application No. EP 0 921 371 A2 for “Method of using a Microscopic Digital Strain Gauge.” Published U.S. application No. 2004/0036853 of Vachon and Ranson discloses a strain gage that can measure strain directly, as well as assess fatigue damage, using a target comprising a strain rosette in the form of a compressed symbol that is defined in terms of its physical dimensions, and more particularly, of the end points of three intersecting line segments that define the strain rosette.
Technical efforts have continued in the area of optical correlation of surface images to detect strain. Specifically, these efforts include, among other things: (1) optical detection of edges of images on surfaces as well as optical detection of edges of surfaces, (2) optical correlation of dot and other geometric shapes applied to surfaces, and (3) optical correlation of the movement of centroids of geometric shapes applied to surfaces. All of these analytical and experimental efforts have been directed to optical detection of strain.
It is to the solution of these and other problems that the present invention is directed.